In rotary wing aircraft, the engines are typically mounted in the top part of the aircraft while the fuel tanks are typically located in the bottom part. During operation, the engine main fuel pump has to lift the fuel from the tank. Gravity and inertial forces acting on the fuel substantially reduce the pressure at the inlet of the engine mounted fuel pump below the fuel pressure in the tank resulting in detrimental conditions for pump suction. The fuel pressure reduces even more when the aircraft flies at altitude, and the ambient air and tank pressures drop. The engine boost fuel pump has to possess exceptional suction capability to be able to induce the fuel from the inlet line at very low inlet pressures. In addition to this effect, due to rapid reduction in fuel pressure, the air, naturally dissolved in the fuel, evolves and travels toward the pump in form of air bubbles. Therefore, the fuel pump, in addition to its ability to induce the fuel at very low pressures, must also be able to induce air-fuel mixture with high air content.
For some rotary wing aircraft applications, the inlet line geometry and the operating conditions act to separate air bubbles from the fuel stream creating a non-homogeneous mixture of air and fuel, which can be in the form of intermittent air bubbles or a relatively large bubble of air. For the boost pump to meet these air handling requirements, the boost pump must be able to compress air. Further, the boost pump must be incorporated into a fuel system that can store the compressed air bubble and can prevent it from reaching the inlet to the main fuel pump.
Industrial applications, i.e. non-aircraft environments, have attempted to meet air pumping requirements by utilizing a side channel liquid ring pump. This type of pump is a hybrid that is able to provide pressures when operating on solid fuel that are on par with regenerative pumps but also has the capability to ingest and compress air.
When pumping air in a liquid ring pump, centrifugal forces separate the fuel and air (or vapor during low suction pressure conditions). The heavier fuel particles are flung to the outer diameter while the air bubbles collect near the impeller hub. A pressure gradient is established with the pressure in the channel at the outer diameter being greater than the pressure at the interior hub. The discharge port is located near the hub, away from the liquid ring.
Due to envelope constraints, such as in helicopters, the inlet and discharge ports may be co-located on one side of the impeller only. With a typical impeller, a non-symmetrical flow pattern results, which allows a pocket of air bubbles to collect on the impeller hub. The compressed air bubbles are carried through the seal zone into the inlet where the bubbles expand proportionally to the discharge/inlet pressure ration. This effect limits both air pumping and suction performance.
Embodiments of the present invention relate to improvements over the current state of the art.